The present invention relates to undersea or transoceanic fiber optic communication cable systems.
Optical fiber segments of undersea optical cable systems are typically joined together sequentially by splicing together successive optical cable ends. This is normally done on board a cable ship prior to submerging the cable, which is an expensive, time consuming, and labor-intensive process. This process is also prone to errors. Once the cable is laid on the sea floor, adjusting or interchanging cable lengths or segments is difficult or impossible.
Optical fiber transmission media suffer from an inherent physical limitation called chromatic dispersion. Chromatic dispersion results from the fact that different wavelengths of light travel through an optical fiber at slightly different speeds, or, more accurately, different optical wavelengths are delayed by different amounts as they pass through the fiber. Because of this phenomenon, an information-bearing light pulse, which will consist of a range of wavelengths, becomes xe2x80x9cspread outxe2x80x9d or xe2x80x9csmearedxe2x80x9d as it travels through a given length of optical fiber. In typical communications systems, many such pulses are transmitted in extremely rapid sequence, with the net result that neighboring pulses can be xe2x80x9csmearedxe2x80x9d together, or overlap, making it difficult for the receiver to distinguish the information contained in each pulse. When the receiver cannot accurately distinguish one pulse from the next, an error condition occurs. If such errors are not corrected, the performance of the optical fiber communications link becomes severely degraded. Since each pulse becomes increasingly xe2x80x9csmearedxe2x80x9d as it travels farther along an optical fiber, the extremely long fiber length used in transoceanic cable systems makes them particularly vulnerable to the errors resulting from dispersion if steps are not taken to mitigate such effects.
In order to reduce the deleterious effects of chromatic dispersion in optical networks, several different types of optical fiber have been developed which have different dispersion properties. Among these, for example, are nonzero-dispersion shifted fiber (NZDF), Lucent Technologies TrueWave(copyright), and Corning""s LEAF(copyright) and SMF-28(copyright) optical fibers. Recently, Lucent Technologies and Corning, among others, have discovered a way of compensating for chromatic dispersion by joining segments of these different types of fiber together in predetermined sequences. This technique is generally known as the xe2x80x9chybrid fiber approachxe2x80x9d to dispersion management, and the combination of different fiber segments is known as a hybrid span.
A hybrid span generally consists of a series of segments of two to three different fiber types joined to the output of each fiber amplifier in a submarine or undersea cable system. Different fiber manufacturers have produced different specifications for the sequences and lengths of fibers in a hybrid span. The hybrid span approach to dispersion management needs to be able to balance overall, or end-to-end, system dispersion, minimize attenuation caused by splice and bend losses, maximize flexibility with respect to in-situ segment length adjustment or tuning, and minimize the costs for manufacture, deployment, and operation of hybrid fiber spans. The current state of the art does not meet these requirements, since fiber lengths are joined by splicing, and a previously spliced cable system does not lend itself to modification once the fiber segments are spliced and laid on the seafloor, and results in a heavy cost burden both for modifying an existing seafloor cable system, or for laying a new hybrid cable system. Once the spliced cable is laid on the sea floor, adjusting the lengths of the different types of fiber segments and/or interchange of segments becomes difficult or impossible, thereby rendering it difficult or impossible to optimize the performance of the cable system. Another problem with the existing cable splicing technique is that fusion splicing of different fiber types creates a new set of problems related to the unique mechanical and optical characteristics of each type of fiber. Thus, splicing of hybrid spans will add a considerable cost burden to the construction of a transoceanic cable system.
It is an object of the present invention to provide a new and improved undersea optical fiber telecommunication system and method permitting use of hybrid dispersion compensation techniques.
According to one aspect of the present invention, a modular undersea optical fiber telecommunication system is provided, which comprises a plurality of cable segments of different fiber types and lengths, each cable segment containing a plurality of optical fibers and having a first end secured to a connector plug unit and a second end secured to a connector receptacle unit, each connector unit containing a plurality of optical fiber contact terminals, each optical fiber in the cable segment being terminated to a respective one of the contact terminals in the plug unit at one end and to a respective one of the contact terminals in the receptacle unit at the opposite end of the cable segment, the plug unit of each cable segment being releasably securable to a receptacle unit of a selected second cable segment and the receptacle unit of each cable segment being releasably securable to a plug unit of a selected third cable segment, whereby the optical fibers in said cable segment are connected with the fibers in the second and third cable segments, and a plurality of the cable segments can be selectively secured together in a predetermined sequence.
The system may also include a plurality of optical amplifier devices or other active or passive optical devices, each device having a connector plug unit at one end and a connector receptacle unit at the other end for releasable connection to a receptacle and plug unit, respectively, of cable segments to be secured in line with the optical device. The connector units may be wet-mateable or dry-mateable, and may be fiber optic connectors or hybrid electrical/optical connectors where the cable system is to carry both optical and electrical signals. If the connector units are wet-mateable, the cable system can be readily modified or tuned after installation on the sea floor, simply by releasing the connector units of both ends of a cable segment to be removed, and replacing it with a cable segment containing a different type of fiber or of a different length, or both, depending on tuning requirements.
According to another aspect of the present invention, a method of installing a suboceanic hybrid optical fiber cable system is provided, which comprises the steps of:
determining a desired sequence of successive telecommunication cable segments of predetermined length, fiber count, and fiber type, whereby at least some cable segments in the sequence are of different fiber types and at least some cable segments in the sequence are of different lengths;
securing the cable segments together end-to-end in the desired sequence by securing a connector plug unit at one end of a first cable segment in the sequence to a connector receptacle unit at one end of a second cable segment in the sequence, securing a connector plug unit at the opposite end of the second cable segment to a connector receptacle unit of a third cable segment in the sequence, and repeating the operation until a desired hybrid sequence is completed; and
installing the hybrid sequence of connected cable segments on the ocean floor to provide a transoceanic cable system.
The cable segments may be secured together on a cable ship and then submerged, or successive segments may be submerged and connected underwater by a remotely operated underwater vehicle. Optical devices such as optical amplifiers may be secured at predetermined intervals in the hybrid sequence, with a hybrid span of two or more cable segments between successive fiber amplifiers. Each optical device may be provided with a plug unit at one end and a receptacle unit at the opposite end for releasable connection between adjacent cable segments.
The method and system of this invention allows a hybrid cable sequence of any desired length to be installed on the ocean floor easily and inexpensively, avoiding all the problems of splicing together different fiber types by fusion, since the modular system uses connectors to establish the physical contact junction between the different fibers, rather than an actual fusion splice. This system also permits a hybrid span to be readily tuned by removing and replacing cable segments simply by releasing the connector units at opposite ends of a segment to be removed, and attaching a new segment in its place using identical connector units at opposite ends of the new segment. Thus, cable segments can be recovered, tuned, and redeployed easily on an as-needed basis. The method and system can be used for new telecommunication cable construction, retrofitting of existing cable spans using hybrid span technology for reducing dispersion, and tuning of hybrid spans for improved results.